Scanning Probe Microscopy (SPM) is a type of microscopy that creates images of surfaces using a probe that scans a specimen. The types of specimens that may be used vary vastly. As an example, a specimen may include a surface of a semiconductor wafer. An image of the surface is obtained by mechanically moving the probe to scan the specimen and recording probe-surface interaction as a function of position.
As previously mentioned, in conventional SPM, as the name implies, an image is obtained by scanning a probe across a sample surface. Typically, there is a fast scan axis (x axis) and a slow scan axis (y axis). As the probe traverses the surface, height (z) measurements of the sample are taken. This is often done via a feedback loop that keeps the spacing between the probe and surface at some constant value. The z measurement is then taken to be the probe displacement (or sample displacement) required to keep this spacing constant. The x and y positions of the probe relative to the sample are also measured as the probe scans; thus an array of (x, y, z) points are obtained and a 3-dimensional (3D) image of the sample surface can be reconstructed.
SPM technology thus requires movement in three degrees of freedom (3 DOF) (x, y, and z). Different manifestations of SPM achieve this in various ways. In moving probe/stationary sample configurations, the probe is attached to a scanner that provides both the x and y-axis scans as well as the vertical height following capability (z axis), while the sample remains stationary. This configuration is illustrated by the schematic diagram of FIG. 1, where a scanner 2, having a probe 4, has three degrees of freedom, and a sample stage 6 is stationary. Alternatively, in stationary probe/moving sample configurations, the opposite is true. Specifically, as illustrated in FIG. 2, the sample stage 6 is capable of providing travel in all three axes while the scanner 2, with probe 4, is stationary. Still further, as illustrated by the schematic diagram of FIG. 3, mixed configurations provide travel capabilities in one or more axes on both the scanner 2, with probe 4, and a sample stage 6.
The primary advantage of SPM is its resolution potential. SPM technologies, such as Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM), can achieve subnanometer resolution. As an example, an AFM can provide nanometer-scale resolution images of a variety of surfaces. The high-resolution capability makes AFMs a promising technology for dimensional metrology of artifacts, such as semiconductor integrated circuits and photomasks. However, many AFMs do not provide sufficiently accurately calibrated axes of measurement. Further, to allow cost-effective use of an AFM in on-line measurement processes, it is necessary to achieve high imaging rates, and thereby high scan rates of an AFM scanning tip.
Unfortunately, scanning a sample surface can take a considerable amount of time, especially when very high resolution is desired. When attempting to scan very rapidly, retaining high resolution and high accuracy are formidable challenges.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.